Known in the art is an internal combustion engine which arranges in an engine exhaust passage an NOX storage catalyst which stores NOX when the air-fuel ratio of exhaust gas is lean and releases the stored NOX when the air-fuel ratio of the exhaust gas is made rich, which arranges in the engine exhaust passage upstream of the NOX storage catalyst a hydrocarbon feed valve, which intermittently makes the air-fuel ratio of the exhaust gas which flows into the NOX storage catalyst rich when the NOX storage catalyst should release SOX, and which uses, as rich control for making the air-fuel ratio of the exhaust gas which flows into the NOX storage catalyst rich, cylinder rich control which generates combustion gas of a rich air-fuel ratio in a cylinder and exhaust rich control which feeds hydrocarbons from the hydrocarbon feed valve so as to make the air-fuel ratio of the exhaust gas rich (for example, see Patent literature 1).
In this regard, when the NOX storage catalyst should release SOX, it is necessary to maintain the temperature of the NOX storage catalyst at the 600° C. or higher SOX release temperature and in that state intermittently make the air-fuel ratio of the exhaust gas which flows into the NOX storage catalyst rich. In this case, if cylinder rich control is used as rich control for making the air-fuel ratio of the exhaust gas which flows into the NOX storage catalyst rich, the oxygen concentration in the exhaust gas which flows into the NOX storage catalyst falls, so the oxidation reaction is weakened and, therefore, the NOX storage catalyst falls in temperature. As opposed to this, if exhaust rich control is used, the engine exhausts a large amount of oxygen and the supplied hydrocarbons react with the large amount of oxygen, so the NOX storage catalyst rises in temperature. Therefore, in this engine, when the NOX storage catalyst falls in temperature, exhaust rich control is used, while when the NOX storage catalyst rises in temperature, cylinder rich control is used. By this, the air-fuel ratio of the exhaust gas which flows into the NOX storage catalyst is intermittently made rich while the temperature of the NOX storage catalyst is maintained at the SOX release temperature.